1. Field of the Invention
This invention relates generally to implantable medical devices, and more particularly to a method and apparatus for limiting unwanted current flow through electrically excitable tissue resulting from application of an external signal on an implanted medical device.
2. Description of the Related Art
The use of implantable medical devices for electrical stimulation of electrically excitable tissue is well known in the medical arts. Electrical stimulation of brain tissue has been used for tremor suppression. Moreover, electrical stimulation of peripheral nerve tissue has been used to promote blood circulation in patients having peripheral vascular disease. Additionally, electrical stimulation of the brain and nerve tissue of the spinal cord has been used for pain management. In such devices, electrodes deliver the stimulation signal to the electrically excitable tissue. The electrodes are operatively connected to an implantable pulse generator which is packaged in a case that is adapted to be implantable. Those electrodes are coupled to that pulse generator by a conductive lead wire.
A user having such an implanted medical device during normal life activities may be forced to go through a time-alternating electromagnetic field. Prevalent examples of sources of electromagnetic field are Electronic Article Surveillance (EAS) systems. Such systems detect theft and are found in the exit doorways of many stores and libraries. EAS systems typically emit an AC electromagnetic field for detecting theft of articles that have an attached electromagnetic tag.
When the user having an implanted medical device walks through an EAS system that emits a significant level of AC electromagnetic field, current may be induced in the electrically excitable tissue by the Faraday effect. According to the Faraday effect, when a conductive loop is disposed within a time-alternating electromagnetic field, a current is induced in that conductive loop. With the implanted medical device within the user, the conductive loop consists of the stimulation electrodes, the lead wire, the implantable pulse generator having a case, and the conductive medium of the body including the electrically excitable tissue between the stimulation electrodes and the case. The case in some implantable medical devices acts as a reference return electrode with respect to the stimulation electrodes and is composed of electrically conductive material.
The user may experience unwanted physiological effects from the current that is induced in the electrically excitable tissue of that conductive loop. This current may cause uncontrolled excitation of electrically excitable tissue which can lead to pain sensations and unwanted motor responses for the user. Thus, means for controlling that induced current is desired.